Electromechanical transducer and method of producing the same

ABSTRACT

An electromechanical transducer includes a substrate, a first electrode disposed on the substrate, and a vibration film including a membrane disposed on the first electrode with a space therebetween and a second electrode disposed on the membrane so as to oppose the first electrode. The first electrode has a surface roughness value of 6 nm RMS or less.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.13/436336, filed Mar. 30, 2012, which claims priority from JapanesePatent Application No. 2011-084673 filed Apr. 6, 2011, which are herebyincorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

Field of the Invention

One disclosed aspect of embodiments relate to an electromechanicaltransducer and a method of producing the transducer. More specifically,an embodiment relates to an electromechanical transducer that is used asultrasonic transducers and a method of producing the transducer.

Description of the Related Art

Electromechanical transducers such as capacitive micromachinedultrasonic transducers (CMUTs) produced by micromachining technologyhave been being researched as substitutes for piezoelectric transducers.These capacitive electromechanical transducers may receive and transmitultrasonic waves with vibration of vibration films.

As a method of producing a CMUT, U.S. Patent Publication No.2005/0177045 describes a method where a cavity is formed by etching of asacrificial layer. In the method described in U.S. Patent PublicationNo. 2005/0177045, in order to prevent an upper electrode from beingetched during the etching of the sacrificial layer, a second electrodeis disposed between a first membrane and a second membrane, and thesacrificial layer is etched.

As in the method described in U.S. Patent Publication No. 2005/0177045,a CMUT is produced by sequentially stacking a lower electrode, aninsulating film, an upper electrode, and a membrane on a substrate. Inthe case of forming a plurality of layers, the thicknesses of the layerstends to vary. If the thicknesses of layers are different among cells orelements, the frequency characteristics among the cells or the elementsvary.

SUMMARY OF THE INVENTION

In the present invention, the variation in frequency characteristicsamong cells or elements may be reduced.

The electromechanical transducer according to aspects of the presentinvention includes a substrate; a first electrode disposed on thesubstrate; and a vibration film including a membrane disposed on thefirst electrode with a space therebetween and a second electrodedisposed on the membrane so as to oppose the first electrode. The firstelectrode has a surface roughness value of 6 nm RMS (root-mean-square)or less.

The method of producing the electromechanical transducer according toaspects of the present invention includes a step of forming a firstelectrode on a substrate; a step of forming a sacrificial layer on thefirst electrode; a step of forming a membrane on the sacrificial layer;a step of forming a second electrode on the membrane; and a step offorming an etching-hole in the membrane and removing the sacrificiallayer through the etching-hole. The first electrode is formed so as tohave a surface roughness value of 6 nm RMS or less.

The present invention may reduce the variation in frequencycharacteristics among cells or elements by optimizing the surfaceconditions of the first electrode.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams illustrating an electromechanicaltransducer to which Example 1 according to aspects of the presentinvention may be applied.

FIGS. 2A to 2G are process diagrams illustrating a method of producingthe electromechanical transducer to which Example 1 of one embodimentmay be applied.

FIG. 3A is a graph showing a relationship between the thickness and thesurface roughness of a first electrode.

FIG. 3B is a graph showing a relationship between the frequencycharacteristics of a vibration film and the surface roughness of a firstelectrode.

FIGS. 4A and 4B are schematic diagrams illustrating an electromechanicaltransducer to which Example 2 according to aspects of the embodimentsmay be applied.

DESCRIPTION OF THE EMBODIMENTS

The present inventors have focused on that the frequency characteristicsof a vibration film vary depending on variation in the thickness of eachlayer formed on a substrate. In particular, the inventors have focusedon that the early step of producing an element, i.e., the step offorming a first electrode, is important. In an electromechanicaltransducer that is produced by stacking a plurality of layers, thelayers that are formed in the steps after the formation of the firstelectrode may have surface shapes reflecting the surface shape of thefirst electrode. From this point of view, the present inventors havefound that there is a certain relationship between the surface roughnessof a first electrode and the frequency characteristics of a vibrationfilm.

Based on this relationship, the embodiment provides a first electrodehaving a surface roughness value of 6 nm RMS or less. An embodiment willnow be described with reference to the drawings. Configuration ofelectromechanical transducer

FIG. 1A is a top view of an element 1 of an electromechanical transduceraccording to aspects of the embodiments, and FIG. 1B is across-sectional view taken along line IB-IB of the cell structure 2surrounded by a dashed line of FIG. 1A. The element 1 of this embodimentincludes a plurality of cell structures 2 that are electricallyconnected to one another. Though FIG. 1A shows only one element, theelectromechanical transducer may have a plurality of the elements. InFIG. 1A, the element 1 is composed of nine cell structures 2, but thenumber of the cell structures is not particularly limited. Furthermore,though the cell structures 2 are arranged in a square lattice form, theymay be arranged in any form such as a zigzag form. The cell structures 2shown in FIG. 1A are circular, but they may be, for example, square orhexagonal. The reference numeral 5 refers to the etching hole.

FIG. 1B is a cross-sectional view of the cell structure 2. The cellstructure 2 includes a substrate 11, a first insulating film 12, a firstelectrode 13, and a second insulating film 14 disposed on the substrate11. The cell structure 2 further includes a vibration film composed of afirst membrane 16, a second electrode 17, and a second membrane 18. Thevibration film is arranged on the second insulating film 14 with aspace, a cavity 15, therebetween. The first membrane 16 is disposed onthe second electrode 17 on the space side (the cavity 15 side) and issupported by a membrane-supporting portion 19. The second membrane 18 isdisposed on the second electrode 17 on the opposite side of the cavity15. The first electrode 13 and the second electrode 17 face to eachother with the cavity 15 therebetween, and a voltage is applied betweenthe first electrode 13 and the second electrode 17 with avoltage-applying unit 50.

The electromechanical transducer may detect an electrical signal fromthe second electrode 17 of each element separately by using lead wiring6. Though the lead wiring 6 is used for extracting the electrical signalin this embodiment, for example, through-wiring may be used. In thisembodiment, both the first electrode 13 and the second electrode 17 aredisposed to each element, but either the first electrode 13 or thesecond electrode 17 may be used as a common electrode. In the case wherean electromechanical transducer includes a plurality of elements, thecommon electrode is electrically connected to all the elements. In alsothis case, the electrical signal of each element may be separatelyextracted as long as either the first electrode 13 or the secondelectrode 17 is separated on an element-to-element basis.

The drive principle of an electromechanical transducer according toaspects of the present invention will be described. In the case ofreceiving ultrasonic waves by the electromechanical transducer, a powersupply unit 50 applies a DC voltage to the first electrode 13 so as tocause a potential difference between the first electrode 13 and thesecond electrode 17. Reception of ultrasonic waves bends the vibrationfilm having the second electrode 17 to change the distance between thesecond electrode 17 and the first electrode 13 (the distance in thedepth direction of the cavity 15), resulting in a change in capacitance.This change in capacitance causes a flow of an electric current in thelead wiring 6. This current is converted into a voltage by acurrent-voltage conversion device (not shown) to give an input signal ofthe ultrasonic waves. As described above, the configuration of the leadwiring may be changed so that a DC voltage is applied to the secondelectrode 17 and that an electrical signal is extracted from the firstelectrode 13 of each element.

In the case of transmitting ultrasonic waves, a DC voltage and an ACvoltage are applied to the first electrode 13 and the second electrode17, respectively, and the electrostatic force vibrates the vibrationfilm. This vibration transmits ultrasonic waves. In also the case oftransmitting ultrasonic waves, the configuration of the lead wiring 6may be changed so that the vibration film is vibrated by applying a DCvoltage and an Ac voltage to the second electrode 17 and the firstelectrode 13, respectively. Alternatively, a DC voltage and an ACvoltage may be applied to the first electrode 13 or the second electrode17 to vibrate the vibration film by electrostatic force. Relationshipbetween frequency characteristics of vibration film and surfaceroughness of first electrode

As described above, in the embodiment, the first electrode has a surfaceroughness value of 6 nm root mean square (RMS) or less. The relationshipbetween the frequency characteristics of the vibration film and thesurface roughness of the first electrode will now be described withreference to FIGS. 3A and 3B. Throughout the specification, the surfaceroughness is measured with an atomic force microscope (AFM) and is shownas roughness root mean square (RMS). The measuring area of the RMS is 5μm×5 μm. The AFM used for measurement is Nanoscope Dimension 3000manufactured by Veeco Instruments Inc. The electromechanical transducerused as the measurement object has the same configuration as theelectromechanical transducer of Example 1 described below except thatthe thickness of the first electrode 13 is changed.

FIG. 3A is a graph showing a relationship between the thickness and thesurface roughness of a first electrode 13 made of titanium. The graphshows the results when Rms was measured by fixing the RF power to 550 Wwhile increasing the titanium thickness from 50 to 200 nm. FIG. 3B is agraph showing a relationship between the frequency characteristics of avibration film and the surface roughness of a first electrode 13. Thegraph shows the results when the frequency characteristics of thevibration film were measured while changing the titanium thickness from50 to 200 nm as in FIG. 3A.

FIG. 3B shows a relationship of the surface roughness and Q value forevaluating the variation of frequency characteristics. The Q value is adimensionless number representing a vibration state and is a valueobtained by dividing the resonance frequency of a vibration film by thehalf bandwidth. A higher Q value means that the frequencycharacteristics of the respective vibration films of the arrayed cellstructures 2 are uniform, that is, the variations among the cellstructures 2 in the shape of the vibration film and the distance betweenthe electrodes are low.

The frequency characteristics were measured with an impedance analyzer4294A manufactured by Agilent Technologies Co., Ltd. The results showthat the Q value is high, such as 200 or more, when the surfaceroughness of the first electrode is 6 nm or less and that the Q valuesharply decreases in the surface roughness range of larger than 6 nm. Inthe range where the Q value is 200 or more, the curve partially has adifferent inclination. This is assumed to be caused by insufficientresolution of the impedance analyzer.

It is understood from FIG. 3B that the surface roughness of the firstelectrode highly affects the variation in frequency characteristics of avibration film and that the Q value considerably changes when thesurface roughness is increased from the range of 6 nm or less to therange of higher than 6 nm. This relationship does not depend on thematerial of the first electrode. From the above, the variation amongcells or elements in frequency characteristics of vibration films may bereduced by controlling the surface roughness value of the firstelectrode to 6 nm RMS or less. Thus, the surface roughness value of thefirst electrode is required to be reduced as much as possible.

In the case of a first electrode made of titanium, the graph shown inFIG. 3A has an inflection point at a titanium thickness of about 100 nm,and the surface roughness sharply increases. Furthermore, the graph hasan inflection point at a titanium thickness of about 200 nm, and theincreasing rate of the surface roughness is reduced. This is probablybecause that in a film forming mechanism, the film-forming surfacetwo-dimensionally grows in a film thickness range of a certain level orless and then suddenly transfers to a three-dimensional growth to growin a mixed state of two-dimensional and three-dimensional growth. Thistendency is observed not only in titanium but also in alloys containingtitanium such as TiW. Accordingly, in the case of using titanium or analloy containing titanium as the first electrode of the embodiment, thethickness may be set to 100 nm or less. Furthermore, in the filmformation, an island form is changed to a thin film form at a thicknessof 10 nm or more, and, consequently, the thickness is 10 nm or more.Thus, the lower limit of the titanium film thickness may be 10 nm ormore. Accordingly, the first electrode according to aspects of theembodiments may have a thickness of 10 nm or more and 100 nm or less.Method of producing electromechanical transducer

The method of producing an electromechanical transducer according toaspects of the embodiment will be described with reference to FIGS. 2Ato 2G, which are process diagrams illustrating a method of producing theelectromechanical transducer shown in FIGS. 1A and 1B. As shown in FIG.2A, a first insulating film 12 is formed on a substrate 11. In the casewhere the substrate 11 is an electrically conductive substrate such as asilicon substrate, the first insulating film 12 is formed for insulatingbetween the substrate 11 and the first electrode 13. Accordingly, in thecase where the substrate 11 is an insulating substrate such as a glasssubstrate, the first insulating film 12 may not be formed. The substrate11 should be a substrate having a surface roughness as low as possible.

Subsequently, as shown in FIG. 2B, a first electrode 13 is formed on thefirst insulating film 12. As described above, the first electrode 13 isformed so as to have a surface roughness value of 6 nm RMS or less. Inthe method of producing an electromechanical transducer by stacking aplurality of layers, the surface roughness of a film is reflected in thesubsequent film formation. Accordingly, it is important to reduce thesurface roughness in the early step. In particular, in metal filmformation, which tends to cause a large surface roughness, a lowersurface roughness is important for preventing a variation incharacteristics. In the embodiment, the first electrode 13 is formedwithin a surface roughness of the above-mentioned range, and thereby thevariation of frequency characteristics of the vibration film isinhibited from increasing even if the surface roughness is successivelyreflected to the layers stacked after this step. The first electrode 13may be made of, for example, titanium or a titanium alloy, which hashigh electrical conductivity, high-temperature tolerance, and highsmoothness.

Subsequently, as shown in FIG. 2C, a second insulating film 14 is formedon the first electrode 13. The second insulating film 14 is formed forpreventing electrical short between the first electrode and the secondelectrode or breakdown when a voltage is applied between the firstelectrode and the second electrode. In the case of driving at a lowvoltage, the second insulating film 14 may not be formed because thatthe first membrane is an insulator. If the second insulating film 14 hasa high surface roughness, the distance between the first electrode andthe second electrode due to the surface roughness varies among the cellsor among the elements. Accordingly, the second insulating film 14 alsoshould be made of a material having a low surface roughness. Forexample, the second insulating film 14 is a silicon nitride film or asilicon oxide film.

Subsequently, as shown in FIG. 2D, a sacrificial layer 25 is formed onthe second insulating film 14. The sacrificial layer 25 is one offactors that determine the shape (depth) of the cavity and thereforeshould be made of a material that is hardly affected by grain boundariesand crystal anisotropy during etching and has high etching selectivityto other constituents. In addition, in order to shorten the etchingtime, the sacrificial layer 25 should be made of a material having ahigh etching rate. Furthermore, the sacrificial layer 25 is required tobe made of a material having a low surface roughness. As in the firstelectrode, if the surface roughness of the sacrificial layer is high,the distance between the first electrode and the second electrode due tothe surface roughness varies among the cells or the elements.Accordingly, the sacrificial layer 25 should be made of a materialhaving a low surface roughness, such as chromium or molybdenum.

Subsequently, as shown in FIG. 2E, a first membrane 16 is formed on thesacrificial layer 25. A membrane-supporting portion is also formed inthis step. The first membrane 16 is required to have a low tensilestress, for example, a tensile stress of higher than 0 MPa and 300 MPaor less. The stress of a silicon nitride film may be controlled byplasma enhanced chemical vapor deposition (PE-CVD) to provide a lowtensile stress. If the first membrane 16 has a compression stress,sticking or buckling may be caused to largely deform the vibration film.The sticking is a phenomenon where the first membrane 16 collapses tothe first electrode 13 side. If the tensile stress is high, the firstmembrane may be broken. Accordingly, the first membrane 16 should have alow tensile stress.

Subsequently, as shown in FIG. 2F, a second electrode 17 is formed, andan etching-hole (not shown) is further formed. Subsequently, thesacrificial layer 25 is removed through the etching-hole to form acavity. The second electrode 17 is required to be made of a materialhaving a low residual stress, high heat resistance, and etchingresistance against the etching of the sacrificial layer. In addition,the second electrode 17 is required to be made of a material that ishardly deteriorated and hardly increases the stress by, for example, thetemperature in the later step, i.e., the formation of the secondmembrane. In the case of a low etching selection ratio, it is necessaryto protect the second electrode 17 when the sacrificial layer is etched,resulting in occurrence of variation. Accordingly, the second electrode17 should be made of a material having etching resistance against theetching of the sacrificial layer. Examples of such a material includetitanium and titanium alloys.

Subsequently, as shown in FIG. 2G, a second membrane 18 is formed. Inthis step, formation of the second membrane 18 and sealing of theetching-hole are performed in the same step. That is, in this step, thesecond membrane 18 is formed on the second electrode (on the surface ofthe second electrode on the opposite side of the cavity), and thereby avibration film having a predetermined spring constant may be formed, andalso a sealing portion that seals the etching-hole may be formed.

In the case where an etching-hole is formed after formation of thesecond membrane 18 and is then sealed, a film for sealing theetching-hole is deposited on the second membrane. Etching for removingthis deposited film causes variations in thickness and stress of thevibration film. On the other hand, in the step of the embodiment, thesealing of the etching-hole and the forming of the second membrane 18are performed in the same step, and thereby the vibration film may beformed only through film-forming steps.

The second membrane 18 is required to be made of a material having a lowtensile stress. As in the first membrane 16, if the second membrane 18has a compression stress, the first membrane 16 may cause sticking orbuckling to largely deform. If the second membrane 18 has a high tensilestress, it may be broken. Accordingly, the second membrane 18 shouldhave a low tensile stress. The stress of a silicon nitride film may becontrolled by PE-CVD to provide a low tensile stress.

Subsequently, lead wiring is formed for easily performing electricalconnection with the first electrode and the second electrode (the stepis not shown). The wiring may be made of a material that has highelectrical conductivity and suitable for assembly, such as aluminum.

In the electromechanical transducer produced by this method, thevariation in the frequency characteristics of the vibration film may bedecreased. In addition, in the electromechanical transducer produced bythis method, the vibration film may be formed only by film-formingsteps. Accordingly, the variation in thickness of the vibration film maybe reduced, and thereby the variations among the cells or elements insensitivity and bandwidth of the electromechanical transducer may bereduced. A preferred embodiment of the present invention

The substrate according to aspects of the embodiments may be anysubstrate such as a semiconductor substrate, a glass substrate, aceramic substrate, or a multiple substrate thereof. In the case wherethe substrate 11 is an insulator such as a glass substrate, the firstinsulating film 12 may not be disposed. In particular, a siliconsubstrate may be used as the substrate 11, and a thermal oxide film maybe used as the first insulating film 12. In particular, a siliconsubstrate having a thermal oxide film may be used as a substrate havinghigh smoothness.

The first electrode 13 may be made of titanium or a titanium alloy. Thesurface roughness of a titanium film used as the first electrode may beprecisely controlled by controlling the RF power of a sputteringapparatus. Since titanium has high heat-resistance, deformation anddeterioration due to high temperature in subsequent steps may beprevented. Surface roughness is reflected in the film formed in the nextstep by the stacking. Accordingly, it is important to reduce the surfaceroughness in the early step.

The second insulating film 14 may be made of silicon oxide. A siliconoxide film formed by PE-CVD has high insulation properties andsmoothness and is excellent in step coverage. Since a high voltage isapplied between the first electrode 13 and the second electrode 17, thesilicon oxide film having a high insulation property and being excellentin step coverage may provide a low surface roughness to the subsequentstep and therefore may be used.

The first membrane 16 and the second membrane 18 may be made of siliconnitride. A silicon nitride film formed by PE-CVD may generally obtain atensile stress. In order to prevent a vibration film from being largelydeformed by a residual stress of a silicon nitride film, a low tensilestress is required. In the electromechanical transducer according toaspects of the embodiments, the second electrode 17 is disposed betweenthe first membrane 16 and the second membrane 18. This configuration mayreduce the distance between the first electrode and the second electrodecompared with that in the case where the second electrode 17 is disposedon the second membrane 18, resulting in an increase in conversionefficiency.

The conversion efficiency herein is the efficiency of convertingvibration of a vibration film into an electrical signal. The conversionefficiency is increased with a decrease in the distance between thefirst electrode and the second electrode. In the case of a vibrationfilm composed of a combination of materials having different thermalexpansion coefficients, the vibration film is warped by a bimetaleffect. However, the stresses may be well balanced by disposing thesecond electrode 17 between the first membrane 16 and the secondmembrane 18 which are made of the same material, and thereby the warp ofthe vibration film may be reduced. As a result, the vibration film maybe prevented from being largely deformed.

The second electrode 17 may be made of titanium or a titanium alloy byelectron-beam evaporation, and a titanium film formed by electron-beamevaporation under a low degree of vacuum may have a tensile stress. Ifthe second electrode 17 is formed with a high compression stress, thestress balance of the second electrode 17 on the first membrane 16 maycause a large deformation of the vibration film, and thereby thevariation in bending of the vibration film is increased. In orderprevent the vibration film from being largely deformed, the secondelectrode 17 should have a low tensile stress. Since titanium has highheat-resistance, deterioration due to high temperature in the step offorming the second membrane may be prevented. In addition, titanium mayreduce the surface roughness, and thereby variation in bending of themembrane may be prevented.

EXAMPLES

The embodiments will be described in detail by using more specificexamples.

Example 1

An aspect of the embodiments will be described with reference to FIGS.1A and 1B. FIG. 1A is a top view illustrating an electromechanicaltransducer of one embodiment, and FIG. 1B is a cross-sectional viewtaken along line IB-IB of FIG. 1A. The element 1 of this Exampleincludes nine cell structures 2.

In FIG. 1B, a cell structure 2 includes a silicon substrate 11 having athickness of 300 μm, a first insulating film 12 disposed on the siliconsubstrate 11, a first electrode 13 disposed on the first insulating film12, and a second insulating film 14 on the first electrode 13. The cellstructure 2 further includes a vibration film composed of a firstmembrane 16, a second membrane 18, and a second electrode 17. The firstmembrane 16 is supported by a membrane-supporting portion 19, and thefirst electrode 13 and the second electrode 17 are arranged so as tooppose to each other with the cavity 15 therebetween.

The first insulating film 12 is a silicon oxide film having a thicknessof 1 μm formed by thermal oxidation. The first electrode 13 is formed oftitanium using a sputtering apparatus so as to have a thickness of 50 nmand a surface roughness of 2 nm as the Rms. The second insulating film14 is a silicon oxide film formed by PE-CVD. The second insulating film14 reflects the surface roughness of the first electrode 13 andtherefore has substantially the same Rms value as that of the firstelectrode 13. The first electrode 13 of this Example is formed on theentire surface of the element 1. In the case where a plurality ofelements are arranged in an array form, an electrical signal may beextracted from each element separately by using the first electrode 13as a common electrode electrically connected to all the plurality of theelements and the second electrode 17 electrically separated on anelement-to-element basis. Alternatively, the second electrode 17 may beused as a common electrode, and the first electrode 13 may be separatedon an element-to-element basis. Furthermore, both the first electrode 13and the second electrode 17 may be separated on an element-to-elementbasis.

The second electrode 17 is formed of titanium using an electron-beamevaporator so as to have a thickness of 100 nm and a tensile stress of200 MPa or less. The first membrane 16 and the second membrane 18 areeach a silicon nitride film formed by PE-CVD so as to have a tensilestress of 100 MPa or less and diameter of 45 μm. The thicknesses of thefirst membrane 16 and the second membrane 18 are 0.4 μm and 0.7 μm,respectively. The second electrode 17 has a diameter of 40 μm. Thecavity 15 has a thickness of 0.18 μm. The thickness of the secondmembrane 18 is about four times that of the cavity 15. Accordingly, theetching-hole may be filled with the insulating film serving as thesecond membrane 18 to satisfactorily seal the cavity 15.

The thickness of the first membrane 16 is smaller than that of thesecond membrane 18, and the spring constant of the membranes is adjustedto a predetermined value by controlling the thickness of the secondmembrane 18. As a result, a vibration film having a predetermined springconstant may be formed only through film-forming steps without etchingthe film serving as the second membrane 18.

The electromechanical transducer of this Example may extract anelectrical signal from the second electrode 17 of each elementseparately by using lead wiring 6.

In the case of receiving ultrasonic waves by the electromechanicaltransducer, the power supply unit (not shown) applies a DC voltage tothe first electrode 13. Reception of ultrasonic waves deforms the firstmembrane 16 having the second electrode 17 and the second membrane 18 tochange the depth of the cavity 15 between the second electrode 17 andthe first electrode 13, resulting in a change in capacitance. Thischange in capacitance causes a flow of an electric current (electricalsignal) in the lead wiring 6. This current is converted into a voltageby a current-voltage conversion device (not shown) to give an inputsignal of the ultrasonic waves.

In the case of transmitting ultrasonic waves, a DC voltage and an ACvoltage are applied to the first electrode 13 and the second electrode17, respectively, and the electrostatic force vibrates the vibrationfilm. This vibration transmits ultrasonic waves.

Example 2

Example 2 according to aspects of the present invention will bedescribed with reference to FIGS. 4A and 4B. FIG. 4A is a top viewillustrating the electromechanical transducer according to this Example,and FIG. 4B is a cross-sectional view taken along line IIB-IIB of FIG.4A. The configuration of the electromechanical transducer of Example 2is the same as that of Example 1 excepting the shape of the firstelectrode. The reference numeral 35 refers to the etching hole.

As shown in FIG. 4B, the cell structure includes a silicon substrate 41having a thickness of 300 μm, a first insulating film 42 disposed on thesilicon substrate 41, a first electrode 43 disposed on the firstinsulating film 42, and a second insulating film 44 on the firstelectrode 43. The cell structure further includes a vibration filmcomposed of a first membrane 46, a second membrane 48, and a secondelectrode 47. The first membrane 46 is supported by amembrane-supporting portion 49. The first electrode 43 and the secondelectrode 47 are arranged so as to oppose to each other with a cavity 45therebetween.

The first insulating film 42 is a silicon oxide film having a thicknessof 1 μm formed by thermal oxidation. The first electrode 43 is formed oftitanium so as to have a thickness of 50 nm and a surface roughness of 2nm as the Rms using a sputtering apparatus. Furthermore, in thisExample, in order to reduce the unnecessary parasitic capacitance formedby the wiring of the first electrode and the wiring of the secondelectrode at positions other than the cavity, the first electrode ispatterned so as to reduce the area where the wiring of the firstelectrode and the wiring of the second electrode overlap each other assmall as possible.

Titanium may be precisely patterned to form the first electrode 43 asshown in the drawing by employing photolithography and etching. A highetching selection ratio to other constituents may be achieved by using asolution containing a perchloric acid as an etchant, and thereby a firstelectrode having notably high smoothness and maintaining the low surfaceroughness may be obtained without deteriorating the peripheralmaterials. Thus, the first electrode 43 and the second electrode 47 haveapproximately the same size, and the cells are connected to one anotherwith a thin wiring 33. The parasitic capacitance may be reduced byarranging the wiring 33 of the first electrode 43 and the wiring 36 ofthe second electrode 47 not to oppose to each other with an insulatingfilm therebetween. The second insulating film 44 is a silicon oxide filmformed by PE-CVD.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. A capacitive electromechanical transducercomprising: a substrate; a first electrode disposed on the substrate;and a vibration film disposed on the first electrode with a spacetherebetween, wherein the first electrode has a surface roughness valueof 6 nm RMS (root-mean-square) or less, wherein the vibration filmincludes a first membrane, a second electrode and a second membrane, andwherein the first membrane is disposed between the second electrode andthe space, and the second membrane is disposed on a surface of thesecond electrode opposite to a surface on which the first membrane isdisposed.
 2. The capacitive electromechanical transducer according toclaim 1, wherein the first electrode is disposed on the substrate with afirst insulating film therebetween.
 3. The capacitive electromechanicaltransducer according to claim 2, further comprising: a second insulatingfilm on the first electrode, wherein the space is formed between thesecond insulating film and the first membrane.
 4. The capacitiveelectromechanical transducer according to claim 1, wherein the firstelectrode comprises titanium or an alloy containing titanium.
 5. Thecapacitive electromechanical transducer according to claim 1, whereinthe first electrode has a thickness of 10 nm or more and 100 nm or less.6. The capacitive electromechanical transducer according to claim 1,wherein the space is formed by etching a sacrificial layer formed on thefirst electrode, after formation of the first membrane.
 7. Thecapacitive electromechanical transducer according to claim 3, whereinthe space is formed by etching a sacrificial layer formed on the secondinsulating film, after formation of the first membrane.
 8. Thecapacitive electromechanical transducer according to claim 1, whereinthe first electrode comprises tungsten.
 9. The capacitiveelectromechanical transducer according to claim 1, wherein the substratecomprises a silicon substrate.
 10. The capacitive electromechanicaltransducer according to claim 2, wherein the first insulating filmcomprises silicon oxide.
 11. The capacitive electromechanical transduceraccording to claim 3, wherein the second insulating film comprisessilicon oxide.
 12. The capacitive electromechanical transducer accordingto claim 1, further comprising a element including a plurality of cellstructures, wherein each of the plurality of cell structures comprisesthe first electrode and the vibration film.
 13. The capacitiveelectromechanical transducer according to claim 2, further comprising aplurality of the elements, wherein an electrical signal from the firstor second electrode is output separately for each of the elements. 14.An apparatus comprising the capacitive electromechanical transduceraccording to claim 1 and a voltage applying unit, wherein the voltageapplying unit applies a voltage between the first and second electrodesso that the electromechanical transducer receives an ultrasonic wave andoutput a current.
 15. An apparatus comprising the capacitiveelectromechanical transducer according to claim 1 and a voltage applyingunit, wherein the voltage applying unit applies a DC voltage to thefirst electrode and applies an AC voltage to the second electrode sothat the electromechanical transducer transmits an ultrasonic wave. 16.An apparatus comprising the capacitive electromechanical transduceraccording to claim 1 and a voltage applying unit, wherein the voltageapplying unit applies a DC voltage to the second electrode and appliesan AC voltage to the first electrode so that the electromechanicaltransducer transmits an ultrasonic wave.
 17. The capacitiveelectromechanical transducer according to claim 1, wherein the vibrationfilm has a surface roughness value of 6 nm RMS (root-mean-square) orless.
 18. The capacitive electromechanical transducer according to claim3, wherein the second insulating film has a surface roughness value of 6nm RMS (root-mean-square) or less.
 19. The capacitive electromechanicaltransducer according to claim 1, wherein the first membrane includessilicon nitride.
 20. A capacitive electromechanical transducer includinga plurality of elements, each of the plurality of elements including aplurality of cells, each of the plurality of cells including a firstelectrode, a second electrode disposed with a space from the firstelectrode, and a first membrane disposed on the second electrode facingthe space, and configured such that a distance between the firstelectrode and the second electrode is changed by reception of anultrasonic wave, wherein the first electrode has a surface roughnessvalue of 6 nm RMS (root-mean-square) or less, and wherein the firstelectrode is a common electrode commonly used by the plurality ofelements.
 21. The capacitive electromechanical transducer according toclaim 20, further comprising a second membrane, wherein the secondmembrane is disposed on a surface of the second electrode opposite to asurface on which the first membrane is disposed.
 22. The capacitiveelectromechanical transducer according to claim 20, wherein the firstelectrode has a thickness of 10 nm or more and 100 nm or less in adirection in which layers are stacked.
 23. The capacitiveelectromechanical transducer according to claim 20, wherein the firstelectrode faces the space via a silicon oxide film, and the firstmembrane includes a silicon nitride film.
 24. The capacitiveelectromechanical transducer according to claim 21, wherein the secondmembrane includes a silicon nitride film.
 25. The capacitiveelectromechanical transducer according to claim 20, wherein the firstelectrode has a surface roughness value of 6 nm RMS (root-mean-square)or less in an area of 5 μm×5 μm.
 26. The capacitive electromechanicaltransducer according to claim 1, wherein the first electrode has asurface roughness value of 2 nm RMS (root-mean-square) or less.
 27. Thecapacitive electromechanical transducer according to claim 20, whereinthe first electrode has a surface roughness value of 2 nm RMS(root-mean-square) or less.
 28. A capacitive electromechanicaltransducer including at least one element, the element including aplurality of cells, the cell including a first electrode, an insulatingfilm disposed on the first electrode, a second electrode disposed on theinsulating film with a space therebetween, and a membrane disposed onthe second electrode facing the space, and configured such that adistance between the first electrode and the second electrode is changedby reception of an ultrasonic wave, wherein the insulating film has asurface roughness value of 6 nm RMS (root-mean-square) or less, andwherein the first electrode is a common electrode commonly used by theelement.
 29. The capacitive electromechanical transducer according toclaim 28, wherein the insulating film has a surface roughness value of 2nm RMS (root-mean-square) or less.
 30. A capacitive electromechanicaltransducer comprising: a substrate; a first electrode disposed on thesubstrate; an insulating film disposed on the first electrode; and avibration film disposed on the insulating film with a spacetherebetween, wherein the insulating film has a surface roughness valueof 6 nm RMS (root-mean-square) or less, wherein the vibration filmincludes a first membrane, a second electrode and a second membrane, thesecond electrode is disposed between the first membrane and the secondmembrane.
 31. The capacitive electromechanical transducer according toclaim 30, wherein the first membrane is disposed on the second electrodefacing the space.
 32. The capacitive electromechanical transduceraccording to claim 30, wherein the insulating film has a surfaceroughness value of 2 nm RMS (root-mean-square) or less.
 33. Thecapacitive electromechanical transducer according to claim 30, whereinthe first electrode comprises titanium or an alloy containing titanium.34. The capacitive electromechanical transducer according to claim 30,wherein the first electrode has a thickness of 10 nm or more and 100 nmor less.
 35. The capacitive electromechanical transducer according toclaim 30, wherein the first electrode comprises tungsten.
 36. Thecapacitive electromechanical transducer according to claim 30, whereinthe insulating film comprises silicon oxide.
 37. The capacitiveelectromechanical transducer according to claim 30, wherein the firstmembrane includes silicon nitride.
 38. The capacitive electromechanicaltransducer according to claim 30, wherein a Q value obtained by dividinga resonance frequency of the vibration film by a half value width of aresonance curve is 200 or more.
 39. The capacitive electromechanicaltransducer according to claim 30, further comprising a current-voltageconversion device configured to convert a electric current caused bychanging a distance between the first electrode and the second electrodeinto a voltage.
 40. The capacitive electromechanical transduceraccording to claim 30, further comprising an element including aplurality of cell structures, wherein the cell structure comprises thefirst electrode and the vibration film.
 41. The capacitiveelectromechanical transducer according to claim 40, further comprising aplurality of the elements, wherein an electrical signal from the firstor second electrode is output separately for each of the elements. 42.An apparatus comprising the capacitive electromechanical transduceraccording to claim 30 and a voltage applying unit, wherein the voltageapplying unit applies a voltage between the first and second electrodesso that the electromechanical transducer receives an ultrasonic wave andoutput a current.
 43. An apparatus comprising the capacitiveelectromechanical transducer according to claim 30 and a voltageapplying unit, wherein the voltage applying unit applies a DC voltage tothe first electrode and applies an AC voltage to the second electrode sothat the electromechanical transducer transmits an ultrasonic wave. 44.An apparatus comprising the capacitive electromechanical transduceraccording to claim 30 and a voltage applying unit, wherein the voltageapplying unit applies a DC voltage to the second electrode and appliesan AC voltage to the first electrode so that the electromechanicaltransducer transmits an ultrasonic wave.